Single-particle Phenomena Research Articles

Seeing is believing: Correlating optoelectronic functionality with atomic scale imaging of single semiconductor nanocrystals.

A unique on-chip method for the direct correlation of optical properties, with atomic-scale chemical-structural characteristics for a single quantum dot (QD), is developed and utilized in various examples. This is based on performing single QD optical characterization on a modified glass substrate, followed by the extraction of the relevant region of interest by focused-ion-beam-scanning electron microscope processing into a lamella for high resolution scanning transmission electron microscopy (STEM) characterization with atomic scale resolution. The direct correlation of the optical response under an electric field with STEM analysis of the same particle allows addressing several single particle phenomena: first, the direct correlation of single QD photoluminescence (PL) polarization and its response to the external field with the QD crystal lattice alignment, so far inferred indirectly; second, the identification of unique yet rare few-QD assemblies, correlated directly with their special spectroscopic optical characteristics, serving as a guide for future designed assemblies; and third, the study on the effect of metal island growth on the PL behavior of hybrid semiconductor-metal nanoparticles, with relevance for their possible functionality in photocatalysis. This work, therefore, establishes the use of the direct on-chip optical-structural correlation method for numerous scenarios and timely questions in the field of QD research.

Nuclear moments of indium isotopes reveal abrupt change at magic number 82.

In spite of the high-density and strongly correlated nature of the atomic nucleus, experimental and theoretical evidence suggests that around particular 'magic' numbers of nucleons, nuclear properties are governed by a single unpaired nucleon1,2. A microscopic understanding of the extent of this behaviour and its evolution in neutron-rich nuclei remains an open question in nuclear physics3-5. The indium isotopes are considered a textbook example of this phenomenon6, in which the constancy of their electromagnetic properties indicated that a single unpaired proton hole can provide the identity of a complex many-nucleon system6,7. Here we present precision laser spectroscopy measurements performed to investigate the validity of this simple single-particle picture. Observation of an abrupt change in the dipole moment at N = 82 indicates that, whereas the single-particle picture indeed dominates at neutron magic number N = 82 (refs. 2,8), it does not for previously studied isotopes. To investigate the microscopic origin of these observations, our work provides a combined effort with developments in two complementary nuclear many-body methods: ab initio valence-space in-medium similarity renormalization group and density functional theory (DFT). We find that the inclusion of time-symmetry-breaking mean fields is essential for a correct description of nuclear magnetic properties, which were previously poorly constrained. These experimental and theoretical findings are key to understanding how seemingly simple single-particle phenomena naturally emerge from complex interactions among protons and neutrons.

Walking droplets, pilot waves, and word choices

After reading John Bush’s article “The new wave of pilot-wave theory” (Physics Today, August 2015, page 47), I want to remind readers of the limitations of the analogy. The oil-drop experiments provide a tangible partial analogue of the pilot-wave picture, but one that is restricted to single-particle phenomena. Such experiments cannot reproduce the types of phenomena that depend on entanglement; only in the case of a single particle does the wavefunction have the same mathematical form—a scalar function over space—as the waves in the oil.Once two particles are involved, the fact that the wavefunction is defined over the configuration space of the system rather than over physical space becomes crucial, and the partial analogy to the oil drops fails. In other words, the analogy does not extend to what most people consider the most interesting aspects of quantum mechanics.© 2016 American Institute of Physics.

Boltzmann conductivity of ferromagnetic graphene with magnetic impurities

We investigate the electrical conductivity of spin-polarized graphene in the presence of short-ranged magnetic scatterers within the relaxation time approximation and the semi-classical Boltzmann approach. Spin-flip scattering of the itinerant electrons from the majority spin sub-band into the minority one results in a minimum in the electrical resistivity at a finite temperature. While this behavior is reminiscent of the renowned Kondo effect, it has an entirely different origin and differs from the Kondo effect in several aspects. In particular, unlike the Kondo effect, this is a single particle phenomena, and it does not require antiferromagnetic coupling between the magnetic moments of impurities and spins of the itinerant electrons.

Charge-carrier injection and transport in organic light-emitting diodes: Single-particle versus mean-field approach

In the framework of the one-dimensional mean-field (MF) drift-diffusion approach the well-defined boundary conditions far away from the metal/insulator contacts of a planar metal/insulator/metal system are used to determine the boundary condition at the interface itself. The novel self-consistent boundary condition linking the carrier density and the electric field at the interface enables a straightforward description of the current voltage (IV) characteristics in forward and reverse bias bridging space charge and injection-limited regimes and accounting for barrier lowering from the potential drop in the used contact materials. Yet, because of the low carrier density in the insulator under injection limitation, single-particle phenomena, such as the Schottky effect, must be considered. We reconsider the validity of the MF approach, depending on the external bias and the prevailing injection barriers. For the crucial parameter window where the MF approach fails and single-particle phenomena become important, a modification of the boundary conditions at the insulator/metal interface is proposed to account for the discrete nature of carriers. The difference between the thus modified MF and the unmodified MF approach is illustrated by several examples.

Time-like flows of energy momentum and particle trajectories for the Klein-Gordon equation

The Klein-Gordon equation is interpreted in the de Broglie-Bohm manner as a single-particle relativistic quantum mechanical equation that defines unique time-like particle trajectories. The particle trajectories are determined by the conserved flow of the intrinsic energy density which can be derived from the specification of the Klein-Gordon energy-momentum tensor in an Einstein-Riemann space. The approach is illustrated by application to the simple single-particle phenomena associated with square potentials.

Phase transitions at zero magnetic field in Si/SiGe quantum wells

A study of the zero magnetic field metal–insulator transition in SiGe quantum well structures and its associated phase diagram reveals the occurrence of two quantum phase transitions in the insulating regime as a function of temperature and carrier density. This is observed as a rapid fall of the resistivity below 400 mK in the insulating phase, followed below 110 mK with a partial recovery and plateau of the resistivity with temperature. A critical carrier density is associated with each one of these transitions, one being 2.2×10 11 cm −2 and the other being 1.8×10 11 cm −2. In this work, we explore the nature of these transitions in terms of collective and single-particle phenomena.

A radiant flow reactor for high-temperature reactivity studies of pulverized solids

Our radiant two-phase flow reactor presents several new possibilities for high-temperature reactivity studies. Most importantly, the thermal histories of the suspension and entrainment gas can be independently regulated over wide ranges. At low suspension loadings, outlet temperatures can differ by hundreds of degrees and gas temperatures are low enough to inhibit hydrocarbon cracking chemistry, so primary products are quenched as soon as they are expelled. With coal suspensions, tars were generated with the highest H/C ratio and lowest proton aromaticity ever reported. Alternatively, particles and gas can be heated at similar rates to promote secondary chemistry by increasing particle loading. Simply by regulating the furnace temperature, arbitrary extents of conversion of coal tar into soot were observed for fixed total mass loss. Under both circumstances heat fluxes are comparable to those in large furnaces, so relevant heating rates and reaction times are accessible. Suspensions remain optically thin even for the highest loadings of technological interest because they are only 1 cm wide. Consequently, the macroscopic behavior remains firmly connected to single-particle phenomena. Mass and elemental closures are rarely breached by more than 5% in individual runs, so interpretations are not subject to inordinate scatter in the data. The reactor is also well suited for combustion studies, as demonstrated by extents of carbon and nitrogen burnout from 50% to 100% for various gas-stream oxygen levels.

Radiation effects in GaAs FET devices

The effects that space and nuclear radiation have on GaAs devices utilized for integrated circuit design are reviewed. The hardness capability of contemporary GaAs devices and logic circuits is presented in terms of four major nuclear and space radiation threat categories: total dose effects, dose rate effects, single particle phenomena, and neutron effects. The experimental test data and theoretical analyses presented demonstrate the tolerance of GaAs discrete JFETs and MESFETs and planar integrated circuits to fast neutron and ionizing radiation, under both transient and cumulative conditions.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Single-Particle Motion in a Large-Orbit Gyrotron

The perturbation on the zeroth-order motion of individual particles in an axis-encircling, large-orbit gyrotron due to a constant-amplitude TE/sup 0//sub ln/ electromagnetic (em) wave is studied analytically and numerically. Single-particle phenomena such as phase bunching and trapping are studied as a function of the frequency difference between the cyclotron motion and the em wave. Analytic solutions are developed for both trapped and untrapped particles and are compared with exact numerical results. The analytic solutions yield the percentage of trapped particles and an expression for the minimum em field necessary for trapping. It is shown that energy loss depends on first-order terms for trapped particles and on second-order terms for untrapped particles. A specific set of parameters is used to display the results.

Angular velocity: a new dimension in nuclei

Nuclei can be studied from their ground states (∼0 ħ) up to angular momenta of order 100 ħ, where they are literally pulled apart by centrifugal effects. This range of angular momenta can be viewed as resulting from ‘cranking’ the nucleus around a ‘rotation’ axis, where the critical variable is the cranking velocity. The calculated response of nuclei to such an imposed angular velocity corresponds well with recent observations, and includes a rich and varied interplay of collective and single-particle phenomena.